Improve code quality

Remove code flaws detected by PMD.

PMD Rules:

* NP_LOAD_OF_KNOWN_NULL_VALUE
* DCN_NULLPOINTER_EXCEPTION

Author: fabiolimace

src/main/java/com/github/f4b6a3/uuid/codec/StandardStringCodec.java

@@ -167,7 +167,7 @@ public String encode(UUID uuid) {
 	public UUID decode(final String string) {
 
 		if (string == null) {
-			throw InvalidUuidException.newInstance(string);
+			throw InvalidUuidException.newInstance(null);
 		}
 
 		final String modified = modify(string);

src/main/java/com/github/f4b6a3/uuid/util/UuidValidator.java

@@ -146,7 +146,7 @@ public static boolean isValid(final char[] uuid, int version) {
 	 */
 	public static void validate(final UUID uuid) {
 		if (uuid == null) {
-			throw InvalidUuidException.newInstance(uuid);
+			throw InvalidUuidException.newInstance(null);
 		}
 	}
 

src/main/java/com/github/f4b6a3/uuid/util/internal/NetworkUtil.java

@@ -103,7 +103,7 @@ public static synchronized String mac(NetworkInterface nic) {
 		}
 
 		try {
-			if (nic != null) {
+			if (nic != null && nic.getHardwareAddress() != null) {
 				byte[] ha = nic.getHardwareAddress();
 				String[] hex = new String[ha.length];
 				for (int i = 0; i < ha.length; i++) {
@@ -112,7 +112,7 @@ public static synchronized String mac(NetworkInterface nic) {
 				mac = String.join("-", hex);
 				return mac;
 			}
-		} catch (SocketException | NullPointerException e) {
+		} catch (SocketException e) {
 			return null;
 		}